1. Volume 48, Issue 5, Pages 979-991.e8 (May 2018)
The Microglial Innate Immune Receptor TREM2 Is Required for Synapse Elimination and Normal Brain Connectivity 
Fabia Filipello, Raffaella Morini, Irene Corradini, Valerio Zerbi, Alice Canzi, Bernadeta Michalski, Marco Erreni, Marija Markicevic, Chiara Starvaggi-Cucuzza, Karel Otero, Laura Piccio, Francesca Cignarella, Fabio Perrucci, Matteo Tamborini, Marco Genua, Lawrence Rajendran, Elisabetta Menna, Stefania Vetrano, Margaret Fahnestock, Rosa Chiara Paolicelli, Michela Matteoli 
Immunity 
Volume 48, Issue 5, Pages e8 (May 2018)
DOI: /j.immuni
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2. Immunity 2018 48, 979-991.e8DOI: (10.1016/j.immuni.2018.04.016)
Copyright © 2018 Elsevier Inc. Terms and Conditions

3. Figure 1
TREM2 Deficiency Affects Microglia Cell Density and Activation
(A) Microglia were counted as Iba-1+ cells per high-power field (HPF) in the CA1 and CA3 hippocampal (HP) regions, dentate gyrus (DG), and cortex (Ctx) in WT and Trem2−/− mice at P18–20. Original magnification, 40X. Scale bar, 50 μm. Zoomed-in inset images: scale bar, 25 μm.
(B) Quantification of microglia densities.
(C–E) Skeletonized reconstruction (C) and quantitative analysis of (D) total branches and (E) junctions per cell in the CA1. For microglia count, WT N = 4, n = 8 fields (CA1, CA3), n = 7 fields (DG), n = 6 fields (Ctx); Trem2−/− N = 6, n= 8 fields. ∗∗∗p < 0.001, unpaired t test. For skeletonize analysis, WT N = 3, n = 23 fields; Trem2−/− N = 3, n = 26 fields. ∗∗p < 0.01, Mann Whitney test. Scale bar, 25 μm. Original magnification, 40X.
(F) Confocal images of CD68+ structures within P2Y12+ microglia in P18–20 WT mice. Scale bar, 25 μm.
(G and H) 3D reconstruction (G) and relative quantification (H) of CD68+ structures per Iba-1 volume. Scale bar, 5 μm. WT N = 5, n = 51 cells; Trem2−/− N = 8, n = 52 cells. ∗∗p < 0.01, unpaired t test.
(I) Microglia were identified as CD11b+CD45low cells upon doublet discrimination and death cell exclusion. Values indicated in FACS plots have to be considered as the relative frequency on the total single live cells.
(J and K) Histograms (J) showing MFIs of surface MHC-II, CD80, CD86, and CD11b and (K) relative quantification. ∗∗p < 0.01, ∗∗∗p < 0.001, unpaired t test. WT N = 3 and Trem2−/− N = 3.
(L) Relative mRNA expression by qRT-PCR of microglia cells sorted from the HP and Ctx of P18–20 WT and Trem2−/− mice. WT N = 2 and Trem2−/− N = 2. ∗p < 0.05, ∗∗p < 0.01, ∗∗∗p < One-way ANOVA followed by Tukey's multiple comparisons test. See also Figure S1.
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4. Figure 2
Neuronal Activity and Synaptic Marker Expression Is Increased in Trem2−/− Mice
(A) Representative traces of mEPSC recordings in CA1 hippocampal acute slices from Trem2−/− and WT mice.
(B and C) Summary graph of mEPSC frequency (B) and amplitude (C). mEPSC frequency: WT 1.739 ± 0.131, N = 5, n = 12 cells; Trem2−/− 2.550 ± 0.09, N = 5, n = 13 cells. mEPSC amplitude: WT 12.8 ± 2; Trem2−/− 14.5 ± 3. ∗∗∗∗p < , unpaired t test. Scale bars, 10 pA, 250 ms.
(D–G) Representative fields (D and E) and relative quantification (F and G) of the CA1 region of P18–20 mice, stained for PSD95 (D and F) and for VGLUT1 (E and G). Scale bar, 10μm. (F) WT N = 5, n = 32 fields; Trem2−/− N = 5, n = 37 fields. (G) WT N = 6, n = 40 fields; Trem2−/− N = 5, n = 65 fields. ∗∗∗p < 0.001, ∗∗∗∗p < , unpaired t test.
(H and I) Western blot analysis (H) and quantification (I) of PSD95, Shank2, and SV2A in the whole hippocampus of P18–20 mice. WT N = 4 and Trem2−/− N = 5. ∗p < 0.05, ∗∗p < 0.005, unpaired t test. See also Figure S1.
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5. Figure 3
TREM2 Is Required by Microglial Cells to Perform Spine Refinement
(A and B) Representative traces of mEPSCs recorded from WT and Trem2−/− postnatal hippocampal neurons in vitro (HNs) (A) and quantification (B). mEPSC frequency: WT 1.639 ± 0.17, n = 15 cells; Trem2−/− 1.731 ± 0.22, n = 15 cells. mEPSC amplitude: WT 21.8 ± 1.3; Trem2−/− 23.9 ± 1.16.
(C) Confocal images and 3D reconstruction of CX3CR1+ microglia cells (red) and GFP-transfected neurons (green) co-cultures. Scale bar, 30 μm.
(D and E) Representative traces (D) and quantitation (E) of mEPSC recordings in WT HNs cultured alone or co-cultured with either WT or Trem2−/− microglia for 24 hr. mEPSs frequency: HN 1.733 ± 0.10, n = 28 cells; HN + WT microglia 1.149 ± 0.10, n = 16 cells; HN + Trem2−/− microglia 1.803 ± 0.12, n = 16 cells. mEPSC amplitude: HN 24 ± 1.2; HN + WT microglia 26.5 ± 2; HN + Trem2−/− microglia 23 ± 2; ∗∗p < One-way ANOVA followed by Tukey's multiple comparisons test. Scale bars, 10 pA and 250 ms.
(F) HNs cultured alone or co-cultured with either WT or Trem2−/− microglia. Scale bar, 10 μm.
(G–I) Analysis of total spine (G), mushroom spine (H), and filopodia (I) density under the conditions described above. Number of total spines/μm, HN 0.6 ± 0.02, dendrites: 63, neurons: 29; HN + WT microglia 0.52 ± 0.02, dendrites: 101, neurons: 41; HN + Trem2−/− microglia 0.63 ± 0.16, dendrites: 94, neurons: 38. Number of mushroom spines/μm, HN 0.41 ± 0.01; HN + WT microglia 0.32 ± 0.01; HN + Trem2−/− microglia 0.43 ± Number of filopodia/μm, HN 0.10 ± 0.01; HN + WT microglia 0.12 ± 0.01; HN + Trem2−/− microglia ∗p < 0.05, ∗∗∗p < 0.001, ∗∗∗∗p < One-way ANOVA followed by Dunn’s multiple comparison test. Bar graphs represent mean ± SEM of 4 independent experiments. See also Figure S2.
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6. Figure 4 TREM2 Is Required for Synapse Engulfment
(A) Confocal and orthogonal images of FM1-43 dye-labeled synaptosomes (Syns) colocalizing with CD68+ structures (white arrows) inside microglia in vitro. Scale bar, 5 μm.
(B) Representative FACS profiles of WT or Trem2−/− CD11b+ microglia populations for FM1-43 dye intensity after the incubation with FM1-43-conjugated Syns for 2 or 24 hr.
(C–D) Engulfment ability of Trem2−/− and WT microglia, calculated by comparing the percentage (C) or the MFIs (D) of CD11b+ cells expressing strong FM1-43 dye intensity. Bar graphs represent mean ± SEM of 3 independent experiments. ∗p < 0.05, ∗∗∗∗p < One-way ANOVA followed by Tukey’s multiple comparison test.
(E) Representative confocal z stack and relative 3D surface rendering showing volume reconstruction of microglia, CD68 and engulfed PSD95, detected within microglial CD68+ structures (yellow arrowheads). On the right, orthogonal view of CD68+ phagolysosomes colocalizing with PSD95. Scale bar, 3 μm.
(F and G) Relative quantification of engulfed PSD95 within CD68 per microglia (F) and total volume of PSD95+ puncta per stack (G). WT N = 5, n = 27; Trem2−/− N = 6, n = 32.
(H and I) Representative images (H) of secondary branches of apical dendrites of WT and Trem2−/− CA1 neurons (P18–20) stained by the Golgi-Cox method and (I) relative quantitation. Number of spines/μm: WT 0.89 ± 0.01, N = 3, n = 110 dendrites; Trem2−/− 1.2 ± 0.02, N = 3, n = 77 dendrites. ∗∗∗∗p < , unpaired t test. Scale bar, 10 μm. See also Figure S2.
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7. Figure 5 Decreased fMRI Functional Connectivity in Trem2−/− Mice
(A) Matrix representation of the whole-brain functional connectome (130 x 130 nodes, 10% sparsity) in the mouse brain, obtained from the average of 20 datasets.
(B) Results of group-difference permutation testing (5,000 permutations, uncorrected) are shown for two contrasts (WT > Trem2−/−and WT < Trem2−/−).
(C) Correction for multiple comparisons using NBS revealed one significant large-scale network (p = 0.011, corrected) that includes connections between prefrontal cortical areas (anterior cingulate, ACA; orbitofrontal, ORB; prelimbic [PL] and retrosplenial [RSP] with hippocampal areas [CA1, DG] and subiculum [SUB; presubiculum, PRE, postsubiculum, POST]).
(D) For this network, a significant reduction in connectivity was found in Trem2−/− mice.
(E) Nodes of the circuit are represented as spheres, with size and color proportional to the numbers of significant under-connected edges. WT N = 9; Trem2−/− N = 10. ∗∗p < 0.01, unpaired t test. See also Figure S3 and Table S3.
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8. Figure 6
P90 Trem2−/− Mice Display Impairments in Repetitive and Social Behaviors
(A–F) P90 mice.
(A and B) Marble burying task: number of buried marbles during 30 min (A) and representative pictures (B) (white arrows indicate not-buried marbles). ∗p < 0.05, unpaired t test. WT N = 13; Trem2−/− N = 14.
(C) Self-grooming test. ∗∗p < 0.01, unpaired t test. WT N = 9; Trem2−/− N = 10.
(D) Percentage of intact material in nestlet shredding. WT N = 9; Trem2−/− N = 10.
(E) Sociability test performed in a social choice paradigm. ∗∗∗p < One-way ANOVA followed by Tukey’s multiple comparison test. WT N = 12; Trem2−/− N = 10.
(F) Social memory test performed in a social novelty paradigm. ∗p < 0.05, ∗∗∗p < One-way ANOVA followed by Tukey’s multiple comparison test. WT N = 12; Trem2−/− N = 10.
(G) P18–20 mice. Juvenile play interaction assessed as time of reciprocal sniffing. ∗p < 0.05, unpaired t test. WT 6 pairs (N = 12); Trem2−/− 5 pairs (N = 10).
(H and I) P90 mice. (H) Morris water maze place navigation task. Each point represents 3 trials. WT N = 22; Trem2−/− N = 16. (I) Spontaneous alternation in the T maze. WT N = 9; Trem2−/− N = 10. See also Figure S4.
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9. Figure 7
TREM2 Protein Is Decreased in 5- to 23-Year-Old Autistic Patients
(A) Representative western blot of fusiform gyrus samples from autistic patients and controls probed with TREM2, β-actin, and GAPDH (glyceraldehyde 3-phosphate dehydrogenase) antibodies.
(B) Quantification of TREM2 levels normalized to β-actin in controls (black circles) and patients (red squares) for the whole group (controls n = 17; ASD patients n = 16) (left) or for subjects included in the age window between 5 and 23 years (controls n = 10, ASD patients n = 11) (right). Unpaired t test: for the whole group, p = ; for the 5–23 age group, ∗p < 0.05 (p = ).
(C) Correlation between ADI-R and TREM2 protein levels. Pearson’s correlation = −0.60, significance (two-tailed) ∗p = ADI-R scores were available for 14 of the 16 ASD patients (red squares). See also Figure S5 and Tables S1, S2, and S4.
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